The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
Vehicles often now include a wide range of features, and increasingly complicated sophisticated electrical and electronic devices. In relation to fuel measurement and monitoring, motor vehicles may include, in addition to a simple fuel gauge, some sort of device or processor for onboard calculation and display of additional data, for example, average fuel consumption, distance that can be driven, either in kilometers or miles, based on a measured amount of fuel remaining in the fuel tank.
Despite these additional features, fuel gauges fundamentally depend upon measurement of fuel level in the fuel tank, and this is often still carried out using relatively simple and imprecise technology. For example, a conventional sender unit in a vehicle fuel tank may consist of a float attached to some sort of rod. This rod is connected to a valuable resistor. The float reflects the fuel level in the tank, and by movement of the rod, the resistance of the variable resistor is varied. The variable resistor is connected in a circuit, including a fuel gauge, so as to give an indication of the fuel level in the tank. Often, this type of sender unit can give quite imprecise feedings at the limits in the fuel tank, i.e. when the fuel tank is nearly full and when it is empty or nearly empty.
For most everyday applications by individual vehicle owners, this is not considered to be a disadvantage. Most drivers and vehicle operators simply need some indication as to the general level of fuel in the fuel tank, so they can make a determination as to when to need fill the vehicle. Indeed, many fuel gauges only give a coarse indication of the fuel remaining in the fuel tank, e.g. many fuel gauges simply indicate fuel tank level by increments of ⅛th of the tank volume. Given that the volume of the tank is often some arbitrary multiple of a standard measurement unit, e.g. a liter or gallon, these increments rarely translates into any exact measure of fuel/volume, for example, two gallons or ten liters. Additionally, sender units measure, in effect, the depth of the fuel in the tank. Unless steps are taken to compensate for variations in the shape of a fuel tank (and this is done in more modern systems with microprocessors) then this is another source of inaccuracy
Given that customers do not expect or require fuel gauges to be very precise, car companies have developed fuel gauges that give a sufficiently accurate indication of fuel level. Additionally, they have released that it is best, when designing fuel gauges, to err on the side of being conservative. For example, vehicle owners like the satisfaction of seeing a full indication on their fuel gauge and would complain if a gauge never registered a full state; so, many sender units and gauges are set up such that, if the tank is anywhere near full, then a full indication is given. It does not bother an individual owner if, as many owners are aware, a vehicle can be driven for, say, 50 to 100 kms, before there is any noticeable indication that the level in the tank has dropped below full. At the bottom end of the tank, sender units are then usually conservatively designed, so that an empty indication is given, even though there may be a substantial amount of fuel, for example a number of liters, still present.
While this design of fuel gauges and sender units may satisfy individual owners and enable fuel gauges and senders to be manufactured inexpensively, it presents problems for the vehicle rental industry. Rental agreements vehicles commonly provide that a vehicle is provided to a renter with a full tank of fuel. The renter is then required to return the vehicle with the tank full. Many rental agreements provide that the rental company will fill up a fuel tank, but usually at rates that are perceived to include a heavy markup over local fuel costs. There is therefore a significant cost incentive for renters to return a vehicle with a full tank.
The problem arises is that, for many vehicles, the vehicle can be driven for some considerable distance, for example, 50-100 kilometers, before there is any indication that a tank is less than full. Additionally, many renters are businessmen and women, who rent vehicles, e.g. at airport locations, but drive relatively short distances. In North America, it is common for businessmen and women to rent a vehicle in airport simply to drive to a hotel, conference, location, etc. In many cases, the total distance they drive would not be sufficient to cause the fuel tank to register any fuel consumed, and the experienced renters are aware of this fact. This encourages experienced renters to return a vehicle without refilling it, being well aware that they may have used up five, ten or more liters of fuel from the tank. Even if the renter does travel some considerable distance, an experienced renter can again fill up the tank well short of the final destination, the return location for the vehicle, knowing that the fuel used for latter part of the journey will still leave the tank showing a full indication.
All of this means that there are considerable additional costs to the rental industry for having to top up fuel tanks to ensure that vehicles are given to customer with a full tank. If vehicles are not topped up, but simply passed on to the next customer, the tank still registers full, but this can be annoyance for the next customer. If the next customer is honest and diligent, drives only a short distance, and then fills up the tank, they can be surprised at the quantity of fuel required to fill the tank. Again, most people in filling a fuel tank of a vehicle will fill it until a pump at a filling station automatically cuts off; it is impractical to judge the amount of fuel in the tank against the actual fuel gauge while filling the vehicle. This leaves the next customer feeling they have been deceived or defrauded by the rental company.